The present invention relates to methods and circuits for control of sensorless, brushless motors.
Recent years have seen great simplification of DC motors, with corresponding benefits in cost and reliability. Historically most DC motors have used brushes to provide current to the correct phase of the rotor windings, and this persists in motors for consumer products; but for products where reliability and lifetime are needed, electronic commutation is now used. By using semiconductor switches (e.g. FETs) to switch current to the appropriate field winding, the need for replacement of brushes, and the attendant ozone generation, production of conductive dust, and potential for sparking, can be avoided.
Initially electronic commutation was usually accomplished by using some other mechanism to sense the physical position of the rotor. The transducers are typically Hall cells mounted at strategic locations in the motor, in order to provide position information for the commutation circuitry. However, the need for these costly components can be eliminated by obtaining motor position information based on the Bemf of the unenergized (floating) winding.
xe2x80x9cBemf,xe2x80x9d or back electromotive force, is the voltage induced on a winding, by the changing magnetic field which is present inside the motor, when the winding is not being electrically driven by the external driving circuit. The proximity of a rotor pole contributes to the changes in the magnetic field (due to the magnetic field in the rotor), and therefore the Bemf provides some information about the instantaneous position of the rotor. Even though the magnitude of the Bemf is highly dependent on the specific motor architecture (and possibly also on the load conditions), a change in the SIGN of the Bemf will occur when a rotor pole passes the center of the floating armature coil. Thus detection of zero-crossings in the BEMF can in principle provide adequate information about rotor position.
FIG. 1 depicts the commutation phase sequencing, rotation phase index, output voltage waveforms, and relevant timing signals. The Bemf of the undriven phase of the motor is an accurate and repeatable reference for the motor phase. By differentially monitoring the voltage across the floating phase, the point at which the voltage is zero, or xe2x80x9czero crossingxe2x80x9d can be established. With this information, timers (analog or digital) are used to commutate (switch to the next winding phase) at a particular angle, normally 20 to 30 electrical degrees after the zero crossing. As can be seen, there is also a large voltage transition during the commutation due to the flyback current of the motor windings. These flyback pulses also make transitions through zero and could cause erroneous indications of a zero crossing.
In order to prevent the flyback pulses from being detected, a masking circuit is used to block any information from entering the Bemf sensing amplifiers during certain times during the cycle.
One possible approach is exemplified by a motor control chip which is designated the L6238. (Other details of this chip are described in the datasheet available from SGS-Thomson Micro-electronics, and also in copending application Ser. No. 08/140,220, filed Oct. 21, 1993, of common inventorship and assignee with the present application, now patent 5,862,301, issued Jan. 19, 1999. Both of these are hereby incorporated by reference.) In operation, the masking circuitry is enabled as soon as a zero crossing is detected, since any additional information detected after this point is redundant until the next commutation cycle. Referring again to FIG. 1, after the commutation phase delay of typically 30 electrical degrees, the older approach continues to mask out any Bemf information for a time equal to 25% of the previous period. This is indicated by the mask pulse signal in FIG. 1. Thus, the total mask time is equal to the time from the zero crossing to the commutation of the motor plus 25% of the previous period or 45 electrical degrees.
For many applications, this scheme has provided sufficient masking of noise including the commutation pulses. With increasing motor speed and subsequent increase in motor current, the commutation current also increases, resulting in commutation current with a longer duration. In addition, there is a continuing trend of slower slew rates to decrease the electrical and acoustical noise. This also increases the time in which the commutation current is present.
FIG. 2A is a waveform derived from a three phase brushless motor driven by the older approach. The upper waveform is the voltage across one of the phases relative to the center tap of a Y-connected motor winding. The lower trace is the output of the amplifier used to monitor the Bemf voltage. In this situation, the period of the 60 degree electrical cycle is roughly 800 xcexcs. The masking pulse after commutation therefore is xc2xc of this time or 200 xcexcs. As can be seen, the slew rate of the output has been decreased to a point where the output voltage is below the zero crossing (Center line) when the masking timeout has completed. Therefore, a false zero crossing is detected BEFORE the true zero crossing occurs. This xe2x80x9cmisfiringxe2x80x9d causes the sequencer to be incorrectly clocked, and the motor spins down due to the incorrect phasing.
The disclosed inventions eliminate this problem by continually monitoring the Bemf, to ensure that the correct sign is present before the mask timer is allowed to time out. In operation, the mask is enabled following the Bemf zero crossing as usual. When commutation to the next phase occurs, the mask pulse counter starts counting down to provide for the additional 15 degrees of masking. In the case of FIG. 2A, if the Bemf crosses the zero point, the mask counter stops counting. Once the commutation current is nearly complete and the voltages rises above the zero level, the counter is again enabled to provide an extended masking signal. After the mask counter times out the masking is disabled in anticipation of the true zero crossing.
In order to ensure that the Bemf sensing window is xe2x80x9copenedxe2x80x9d at the correct time, the scheme described can be modified to continue the mask countdown for some predetermined count, i.e. 7.5 electrical degrees before an improper Bemf polarity can disable the counter.
The invention adds the polarity detector to determine the polarity of the Bemf signal. The polarity of the Bemf now determines whether or not the masking counter is allowed to count, ensuring that the commutation pulse is completely masked out under all conditions.
Many publications have discussed the problems of sensorless brushless DC motor control, including e.g. Pouilloux, xe2x80x9cFull-wave sensorless drive ICs for brushless DC motors,xe2x80x9d 10 ELECTRONIC COMPONENTS and APPLICATIONS 2 (1991); Antognini et al., xe2x80x9cSelf synchronisation of PM step and brushless motors; a new sensorless approach,xe2x80x9d in ACTUATOR 90: PROCEEDINGS OF 2ND INTERNATIONAL TECHNOLOGY-TRANSFER CONGRESS at 44 (ed. K. Lenz 1990); Bahlmann, xe2x80x9cA full-wave motor drive IC based on the back-EMF sensing principle,xe2x80x9d 35 IEEE TRANSACTIONS ON CONSUMER ELECTRONICS 415 (1989); Paraskeva et al., xe2x80x9cMicroprocessor control of a brushless DC motor,xe2x80x9d in PROCEEDINGS OF THE CONFERENCE ON DRIVES/MOTORS/CONTROLS 84 at 80 (1984); U.S. Pat. No. 5,343,127 of Maiocchi, xe2x80x9cStart-up Procedure for a Brushless, Sensorless Motor;xe2x80x9d U.S. Pat. No. 5,319,289 of Austin et al., xe2x80x9cAdaptive Commutation Delay for Multi-pole Brushless DC Motors;xe2x80x9d U.S. Pat. No. 5,202,616 of Peters et al., xe2x80x9cBipolar or Unipolar Drive Back-EMF Commutation Sensing Method;xe2x80x9d Duane Hanselman, BRUSHLESS PERMANENT-MAGNET MOTOR DESIGN (1994); and T. J. E. Miller, BRUSHLESS PERMANENT-MAGNET AND RELUCTANCE MOTOR DRIVES (1993); all of which are hereby incorporated by reference.